Objective of the proposed research is to elucidate the biochemical mechanism involved in active cation transport by the normal kidney, and in certain pathologic conditions characterized by altered renal handling of sodium and potassium. We will evaluate the activity, properties and physiologic role of a sodium-stimulated, ouabain-insensitive ATPase in kidneys from several mammalian species. This newly identified enzyme may be the biochemical equivalent of an alternative mode of sodium extrusion different from that associated with the classic sodium pump. In particular we will study a possible role of this "second-pump ATPase" in the modulation of acute changes in tubular sodium reabsorption, which appear to be mediated by mechanisms that do not involve the Na-K-ATPase enzyme system. To obtain a better understanding of the relationship between Na-K-ATPase and other transport ATPases on the one hand and renal function on the other we will develop micromethods for the determination of these enzymes in single nephron segments. These techniques will make it possible to measure the enzymes in tubule fragments in which the spatial orientation of the cells is preserved, and at the same time allow the precise localization of the function under study. To evaluate the mechanisms of sodium adaptation in renal failure, plasma and extracellular fluid volumes, renal hemodynamics and sodium handling, Na-K-ATPase, and "second-pump ATPase" will be measured in a uremic animal model developed by us. In this model uremia is produced by urinary diversion into the circulation without altering renal morphology, and therefore function and biochemistry can be evaluated in structurally intact kidneys existing in a controlled uremic environment.